1. Field of the Invention
The present invention relates to an image forming apparatus based on, for example, an electrophotographic scheme, and a density adjusting method for the apparatus.
2. Description of the Related Art
For color image forming apparatuses that print color images on the basis of an electrophotographic scheme, a process of adjusting the image forming apparatus (this process is hereinafter referred to as an image adjusting process) needs to be executed in a sequence different from an image forming process of actually forming images, in order to stabilize the quality of formed (printed) images. Control for the image adjusting process includes an ATR (Automatic Toner Refresh) patch detection process of making the color perception of formed images constant and ATVC (Automatic Transfer Voltage Control) that allows a toner image formed on a photosensitive member to be appropriately transferred to a paper or a transfer member.
The ATR patch detection process is control which makes the color perception of formed images constant and which maintains the fixed concentration ratio of toner to a carrier (developing material) in a developer. This control periodically supplies a developer with toner the amount of which is equal to that of toner consumed for image formation. Toner supply is controlled by forming a patch image on a photosensitive member or a transfer member so that a photo sensor placed opposite the patch image formed detects reflected light from the patch image to determine the concentration ratio of the developing material to the toner.
Image forming apparatuses such as printers and copiers transfer a toner image formed on a photosensitive drum that is a photosensitive member or on an image carrier to a print sheet (transferred member) such as a sheet of paper or an intermediate transfer member. On this occasion, a transfer member such as a transfer roller is abutted against the photosensitive drum to form a transfer nip (transfer site). A transfer bias is then applied to the transfer member with the print sheet passed through the transfer nip. This allows the toner image on the photosensitive drum to be transferred to the print sheet. The transfer roller, serving as the transfer member, normally has its resistance value appropriately adjusted by dispersing conductive particles in an elastic member such as rubber or sponge. However, the resistance value of such a transfer roller varies significantly as a result of a manufacturing variation, an environmental variation, or the lifetime. This makes it difficult to offer high transferability through stable application of the transfer bias.
Ideally, the amount of electric charge applied to the back surface of the print sheet is appropriately controlled in order to offer constant high transferability. To achieve this, for example, the transfer roller may be controllably subjected to a fixed current. However, the passage width (the width of print sheets perpendicular to a conveying direction) of print sheets for the image forming apparatus is not fixed. The width of a part of the transfer roller which directly contacts the surface of the image carrier thus varies depending on the width of print sheets used. This causes the load impedance of the transfer roller with respect to the surface of the image carrier to vary between a part of the transfer roller which contacts the print sheet and a part which does not contact the print sheet. Particularly in an area in which no print sheet is present (the drum or the intermediate transfer member directly contacts the transfer roller), the load impedance is so small as to allow a large current to flow in a concentrated manner. This may result in low transferability in an area in which the print sheet is present.
To eliminate such a disadvantage of the simple constant current control, an ATVC scheme has been proposed. This scheme passes a given current through the transfer roller with no print sheet at the transfer nip and records a generated voltage required for the transfer; the given current is determined by assuming a current passed through the transfer roller during a transfer operation. During actual transfer, a corrected voltage is applied which is equal to the generated voltage, the generated voltage multiplied by a coefficient, or the generated voltage to which a constant is added. However, the ATVC scheme requires a constant current circuit, which increases costs. Moreover, the ATVC scheme employs a hardware configuration with a capacitor as means for storing an output voltage during a constant current operation. Thus, the output voltage during transfer may be affected by a variation in capacitor voltage caused by leakage, the tolerance of gain resistance, or a variation in temperature characteristics. Further, the ATVC scheme is implemented using hardware. As a result, constants, for example, a constant current value and coefficients required to correct the generated voltage to the appropriate transfer voltage are determined in a stage of a circuit design of the image forming apparatus. Thus, the ATVC scheme is disadvantageously limited to the simple bias control.
To eliminate this disadvantage, a software-based ATVC scheme has been proposed which uses means for digitally increasing or reducing the voltage applied to the transfer roller, means for detecting a current flowing from the transfer roller into the image carrier, and means for determining whether or not the current flowing from the transfer roller into the image carrier has reached a desired value (target current). This scheme enables the current flowing from the transfer roller into the image carrier to converge to a given value to achieve control equivalent to that of the constant current circuit in the hardware-based ATVC scheme. The software-based ATVC scheme applies a transfer bias step by step and detects a current flowing from the transfer roller into the image carrier. When the current flowing from the transfer roller into the image carrier reaches the target current value, this control is ended. The transfer bias is then stored in a RAM or the like so as to be applied during the following transfer. However, this ATVC scheme requires the output voltage to be repeatedly varied step by step until the current flowing from the transfer roller into the image carrier reaches the given value. This disadvantageously increases control time. If the circumferential resistance of the transfer roller varies markedly as a result of a manufacturing error, the current at each output voltage is desirably determined by averaging the current values obtained during at least one rotation of the transfer roller. If the current detecting circuit operates under a state of heavy noise, the current at each output voltage is desirably more frequently sampled for averaging. However, such an averaging process further increases the control time.
The above ATR patch detection process and ATVC process are adjustive control required to allow the apparatus to output stable images. However, during the execution of the ATVC, the current flowing from the transfer roller into the image carrier needs to be monitored with the transfer voltage varied until the current converges to the given target value. Thus, an attempt to control ATR patch detection during the ATVC may cause a patch image for the ATR patch detection control to be affected by a variation in transfer voltage based on the ATVC. This may lead to incorrect density corrections. Thus, these control operations needs to be sequentially executed.
In short, the conventional system must sequentially execute the ATR patch detection process and ATVC process at different timings; both the ATR patch detection process and ATVC process are adjustive control required to stabilize images. Thus, the duration of the adjustments equals the simple sum of the control times of the ATR patch detection and the ATVC process. This may disadvantageously degrade productivity for users.
Japanese Patent Laid-Open Nos. 2001-166553 and 2002-014505 disclose the simultaneous execution of image density correction and auto registration correction. However, these documents do not teach the image density correction executed in parallel with the ATVC.